It is described that an .alpha.-glucosidase inhibitor inhibits an .alpha.-glucosidase which is localized at a fine villus in a small intestine, and controls a rapid increase in blood sugar after meal and next increase in insulin level (Diabate Medicine, 10, 688 (1993)). Since it suppresses metabolism of a carbohydrate (especially oligosaccharide derived from starch, sucrose and the like) also in human and the other animals, and exhibits an inhibitory effect of blood sugar increase, it is effective in improving a hyperglycemic condition as well as various diseases induced by a hyperglycemia such as obesity and diabetes. And a food product obtained by adding .alpha.-glucosidase thereto is a suitable meal for a patient having a symptom and also for a healthy human on the purpose of prophylaxis against such symptom.
As an .alpha.-glucosidase inhibitor derived from a food, for example, a enzymatic hydrolysate of an animal protein or a vegetable protein were disclosed in Japanese Unexamined Patent Publication No. 9-65836(1997) and tea polyphenol was disclosed in Japanese Unexamined Patent Publication No. 5-17364(1993).
However, the .alpha.-glucosidase inhibitor disclosed in Japanese Unexamined Patent Publication No. 9-65836(1997) had a problem that it should be taken in a large amount as a food for exerting its activity. And the .alpha.-glucosidase inhibitor disclosed in Japanese Unexamined Patent Publication No. 5-17364(1993) had problems that it was complicated to purify polyphenol and it was necessary to take a large amount in case of routinely consumed tea. An object of the present invention is to provide an excellent .alpha.-glucosidase inhibitor, which has a high activity and can readily be ingested.